Since the advent of PCM pulse transmission systems, an attempt has been made to use the existing network telephone cables to transmit a given number of communications. It then became essential to know certain characteristics of these cables, such as the impedance, the attenuation of pairs, the cross-talk between pairs for frequencies approaching the Nyquist value.
In particular, in order to determine the percentage of pairs that could be used in a cable, we have proceeded to systematic measurement of near-end and far-end cross-talk attenuation.
The article entitled "Mesure en regime numerique de la diaphonie sur des caables apaires symmetriques" by J. Boulvin, published in the French technical journal "Commutation et Transmission", 29.sup.e A., n.sup.o 2, April 1975, pp. 194-208 describes two types of measurements: one using a disturbing generator with sinusoids of variable frequency, the other in which random digital sequences are used.
In accordance with the above mentioned aritcle, we transmit digital pseudo-random sequences, simultaneously on a number of disturbing pairs and we measure the effective noise voltage received by a receiver connected at one end of a pair being observed. This measurement receiver is made up of a correcting amplifier similar to that of a regenerating repeater. The output of the correcting amplifier is connected to a wide band rms voltmeter.
In practice, the measuring instrument described in the above mentioned article can be used in particular to test special symmetric pair cables used only for 8.448 Mbit/s digital transmission systems. In these cables which are used only for digital transmission, the principal source of noise is effectively cross-talk whose effect is equivalent to, with sufficient precision, the superposition of a Gaussian noise on the useful signal. Furthermore, since these cables are provided for heavy traffic, the Gaussian noise is practically stationary.
The introduction of telematics now faces challenges that occur mainly due to certain limitations of the existing telephone networks. Concerning this subject, we can refer to the French technical journal "L'echo des recherches", n.sup.o. 111 of the first trimester 1983, which was dedicated to the presentation of a "network integrating telephone and data", called RITD network. In this future network, we will use subscriber telephone cable pairs to transmit data corresponding to rates that could reach 2.048 Mbit/s. Within the same cable, in proximity to the digital transmission pairs, subsist pairs responsible for analog telephone traffic. We know that conventional analog communication signals generate disturbance of an impluse noise nature, such that the measurement time described in the above mentioned article, which is sufficient for a stationary Gaussian noise, is no longer adequate.